The present invention relates to an optical network unit registration method in a network configured from a plurality of optical line terminals and a plurality of branches including an optical network unit.
A communications network linking a building (station) owned by a communications operator and a subscriber's home is called an access network. In response to the increases in communication capacity of recent years, optical access networks that enable the transmission of very large amounts of information by utilizing optical communication are becoming mainstream.
One example of an optical access network is a passive optical network (PON). A PON is configured from one optical line terminal (OLT) provided in the station, a plurality of optical network units (ONU) respectively provided in subscribers' homes, and an optical splitter. The OLT, ONUs, and the optical splitter are connected by an optical fiber.
A single optical fiber is used for the connection between the OLT and the splitter. This single optical fiber is shared by a plurality of ONUs. Further, the optical splitter is a low-cost passive element. Thus, PONs have excellent economic performance, and they are also easy to maintain. Consequently, the introduction of PONs is rapidly proceeding.
In a PON, the signals transmitted from the respective ONUs to the OLT (hereinafter, sometimes referred to as “uplink optical signals”) are multiplexed by the optical splitter, and transmitted to the OLT. On the other hand, the signals transmitted from the OLT to the respective ONUs (hereinafter, sometimes referred to as “downlink optical signals”) are demultiplexed by the optical splitter, and transmitted to the respective ONUs. Further, to prevent interference between the uplink optical signals and the downlink optical signals, the uplink optical signals and the downlink optical signals are allocated respectively different wavelengths.
Further, in PONs, various multiplexing technologies are used. Examples of multiplexing technologies used in PONs include time division multiplexing (TDM), in which short time divisions on the time axis are allocated to each subscriber, wavelength division multiplexing (WDM), in which different wavelengths are allocated to each subscriber, code division multiplexing (CDM), in which different codes are allocated to each subscriber and the like. Among these multiplexing technologies, a TDM-PON utilizing TDM is currently the most widely used.
In a TDM-PON, TDMA (time division multiple access) is used. TDMA is a technology in which the OLT controls so that uplink optical signals from different ONUs do not collide with each other by managing the transmission timing from each ONU.
A PON system that uses Ethernet® technology is called Ethernet®-PON, and a system that uses Gigabit (1×109 bit/sec) Ethernet® technology is called GE-PON. GE-PON is standardized by IEEE 802.3ah.
In a GE-PON system, to perform communication between the OLT and an ONU, the ONU has to be registered in the OLT. In a GE-PON system, since a plurality of ONUs are connected, registration of a new ONU needs to be performed without affecting communication between the other registered ONUs and the OLT. Consequently, the above-mentioned IEEE 802.3ah (hereinafter referred to as the “standard”) stipulates the procedure for detecting and registering an unregistered ONU by the OLT (hereinafter referred to as a “discovery sequence”).
The OLT periodically broadcasts a discovery gate. A discovery gate is transmitted to all the ONUs regardless of whether the ONU is registered or not. When an ONU newly connected to the PON system is turned on and is capable of receiving signals, it periodically receives a discovery gate.
When an unregistered ONU receives a discovery gate, the ONU transmits a register request to the OLT requesting registration. The register request includes a MAC address as an individual identification number of each ONU.
On the other hand, at the OLT, a discovery window is set. During the period that this discovery window is open, the OLT waits for reception of a register request.
When the OLT receives a register request, the OLT recognizes the ONU MAC address. The OLT then transmits a register to the ONU that has the recognized MAC address. The register includes a link number (LLID) for the PON system.
After transmitting the register, the OLT notifies the transmission band and the transmission timing, and transmits a gate permitting transmission of uplink optical signals to the ONU.
The ONU that received the gate transmits a register acknowledgement (ACK) to the OLT. When the OLT receives the register acknowledgement, the ONU registration is complete. Namely, the discovery sequence is finished (e.g., refer to JP 2010-278525A).
After the ONU is registered, normal OLT-ONU communication is performed.
Normally, in a PON system, one OLT manages a branched optical transmission path and one PON branch that includes the ONUs connected to a branch destination of this optical transmission path. For example, if there are only a few ONUs included on one PON branch, the OLT is shared among few ONUs, which is expensive. Therefore, when there is only a few ONUs on a PON branch, it is desirable if one OLT manages a plurality of PON branches.
To achieve this, a PON system has been proposed that enables a plurality of PON branches to be managed by one OLT by using TDM and WDM together (hereinafter referred to as a “TDM/WDM-PON”.
A TDM/WDM-PON is configured from one or a plurality of OLTs, an optical routing unit, and a plurality of PON branches connected to the OLT(s) via the optical routing unit.
The optical routing unit has a plurality of optical communication ports. The plurality of optical communication ports are divided into a first group connected to the OLT, and a second group connected to the optical transmission path of a PON branch. An optical signal input to an optical communication port of one of the groups is output from an optical communication port of the other group determined based on the wavelength of that optical signal.
By having such a configuration, in the TDM/WDM-PON, one OLT can transmit downlink optical signals to different PON branches by changing the transmission wavelength of the downlink optical signals. Further, the ONUs can transmit an uplink optical signal to a specific OLT by transmitting an uplink optical signal having a wavelength instructed from the OLT. Consequently, one OLT can communicate with an arbitrary PON branch configuring the TDM/WDM-PON.